An ultrasonic enhanced-contrast imaging method and device using an ultrasonic contrast medium are often used for measuring the blood flow within tissue. An example of such an imaging method and device are described in “Ultrasound Contrast Imaging: Current and New Potential Methods: Peter J. A. Frinking et al.” in “Ultrasound in Medicine & Biology”, Vol. 26, No. 6, p. 965, 2000.
The ultrasonic contrast medium is generally formed by mixing many air bubbles into a liquid medium, such as a physiological salt solution, etc. For example, the ultrasonic contrast medium is formed by covering an inert gas (C3F8, C4F10) with a protein film or a fat film and is generally formed in a spherical shape. The particle diameter distribution of the ultrasonic contrast medium is generally set to a Gaussian normal distribution, and the average particle diameter is several μm. However, in air bubbles of 0.5 μm or less, the air bubbles gather, and become substantially larger diameter particles, so that the normal distribution is slightly distorted.
Such a contrast medium is generally injected through a vein into the organism. When an ultrasonic beam is irradiated to a contrast medium that has been injected into the organism and its sound pressure is low, the contrast medium is deformed, and acoustic information created by this deformation is reflected and emitted from the contrast medium as a response signal of the ultrasonic wave. In contrast to this, when the sound pressure is high, the contrast medium is destroyed, and a strong response signal is emitted from the contrast medium due to this destruction. In each case, the ultrasonic contrast medium exhibits a nonlinear response to the ultrasonic wave. When the ultrasonic wave, whose fundamental frequency component is f0, is irradiated, the signal of a higher harmonic wave component 2f0 is included in the response signal, in addition to the signal corresponding to the fundamental frequency component f0.
Such behavior of the contrast medium as deformation and destruction is generally divided into an initial time phase and a latter time phase, depending on the time that has passed since the injection of the contrast medium through the vein. The initial time phase is the time phase in which the ultrasonic contrast medium injected through the vein flows by blood circulation into the tissue, such as the liver, etc., which represents the diagnostic object. The latter time phase is a time phase in which it is anticipated that the ultrasonic contrast medium that has flowed and been distributed into the tissue is has now sufficiently flowed in reverse out of the tissue with the blood circulation after 2 to 8 minutes have passed after the injection of the contrast medium through the vein. In the initial time phase, an ultrasonic sound pressure (e.g., MI: mechanical index=0.2) for generating a sufficiently higher harmonic wave, without destroying the contrast medium, is generally used. When the higher harmonic wave component 2f0 included in the response signal from the contrast medium is detected, it is possible to grasp the distribution and flow of the contrast medium in the tissue and blood vessels. In the latter time phase, the contrast medium will have almost all flowed out of the tissue, but one portion of the contrast medium is trapped within the tissue. A diseased portion and a healthy normal portion of the tissue differ as to whether the contrast medium is trapped in the tissue or not. When an ultrasonic wave having a high ultrasonic sound pressure (e.g., it is said that MI is about 0.8 or more) capable of destroying the contrast medium is irradiated in this latter time phase, a strong reflection signal is generated in the course of destruction of the contrast medium. Accordingly, it is possible to discriminate the area where the contrast medium is trapped, i.e., the diseased portion and the area where the contrast medium was not trapped, i.e., the healthy normal portion, by detecting the higher harmonic wave component 2f0 included in the response signal from the contrast medium.
The ultrasonic enhanced-contrast imager is a device for detecting the higher harmonic wave component 2f0 included in the response signal from the contrast medium and then imaging the blood flow distribution and the diseased portion within the tissue based on the position of the contrast medium. Therefore, the 2f0 component is conventionally extracted, and the existence of the contrast medium is detected by using a relatively narrow band pass filter (e.g., 1.8f0 to 2.2f0) having 2f0 as a central frequency. Namely, since the existence of the 2f0 component corresponds to the existence of the contrast medium, the largeness and smallness of the 2f0 component indicates the spatial density distribution or the destruction of the contrast medium. Accordingly, it is possible to detect into which part of the tissue the contrast medium has flowed, and in which part the contrast medium is trapped. In this case, since the frequency band is narrow, there arises the problem that the depth resolution is deteriorated.
In contrast to this, methods for extracting the higher harmonic wave by utilizing a non-linearity with respect to the frequency of the contrast medium response signal, without using a band pass filter, have been proposed in U.S. Pat. Nos. 5,632,277 and 5,706,819. In accordance with these methods, an ultrasonic pulse based on a first ultrasonic signal is irradiated into the organism, and its response signal is received. Then, an ultrasonic pulse based on a second ultrasonic signal obtained by inverting the polarity of the first ultrasonic signal is irradiated in the same ultrasonic beam direction at a short time interval, and its response signal is received. The component corresponding to the fundamental wave frequency f0 within the response signal from the contrast medium is effectively removed by adding these received signals, and the higher harmonic wave component 2f0 is emphasized. Thus, the contrast medium can be detected with high depth resolution without using a band pass filter.
Further, JP-A-2000-300554 proposes a method wherein a first ultrasonic signal has a waveform in which a period t1 providing a signal level of a positive constant value and a period t2 providing a signal level of a negative constant value are repeated, and a second ultrasonic signal has a waveform obtained by inverting this first ultrasonic signal with respect to the time axis. In accordance with this construction, the symmetry of an ultrasonic pulse based on the first and second ultrasonic signals is raised, and the signal of a fundamental wave component (linear component) can be lessened.
Each of these conventional techniques is effective to extract or emphasize the higher harmonic wave component 2f0 caused by the contrast medium. However, no consideration has been given to the case in which the higher harmonic wave component 2f0, that is included in the response signal from the tissue, is large to such an extent that this higher harmonic wave component 2f0 cannot be neglected in verification of the higher harmonic wave component included in the response signal of the contrast medium. Therefore, there are cases in which the higher harmonic wave component included in the response signal of the contrast medium can not be effectively extracted, such as where the tissue is relatively deep beneath the body surface.
Namely, a nonlinear phenomenon, which here is the key to contrast medium detection, is also caused by propagating the ultrasonic wave within the tissue in addition to the contrast medium. In this case, the higher harmonic wave component 2f0, having a frequency twice the fundamental frequency f0 of the irradiated ultrasonic wave, is also generated. In particular, the strength of the signal of the higher harmonic wave component 2f0 included in the response signal from the tissue is increased as the depth is deepened, i.e., as the propagation length of the ultrasonic wave is increased. Therefore, when the higher harmonic wave component 2f0 of the tissue response signal is equivalent to or larger than the higher harmonic wave component 2f0 included in the response signal of the contrast medium, the higher harmonic wave component 2f0 of the tissue response signal prevents the detection of the contrast medium.
For example, the higher harmonic wave component of 2f0 is emitted from both the contrast medium within the blood vessel buried into the tissue, such as in a blood vessel within the liver, and from the tissue, during the detection of the contrast medium. Therefore, there is a fear that the existence of the contrast medium will be erroneously detected. Namely, in the conventional technique for emphasizing the higher harmonic wave component of 2f0, the 2f0 component included in the response signal from the contrast medium can not always be discriminated from the higher harmonic wave component 2f0 from the organic tissue. Accordingly, there is a case in which the detecting accuracy of the higher harmonic wave component of the contrast medium is reduced, and the definition of an enhanced-contrast image cannot be improved.
FIGS. 2A and 2B are graphs which shows the result of a detailed examination of the nonlinear response of the contrast medium and the tissue with respect to the ultrasonic irradiation of the fundamental frequency 2f0. These graphs typically show a frequency spectrum of the reflection response signal when the ultrasonic wave of the fundamental wave component f0 is irradiated to the contrast medium distributed into the tissue. The axis of abscissa shows a frequency normalized at the fundamental wave f0, and the axis of ordinate shows the signal strength of each frequency component. FIG. 2A shows the response signal from a relatively shallow part near a probe. FIG. 2B shows the response signal from a relatively deep part far from the probe. As can be seen from these figures, in both the shallow and deep parts, the response signal 1 of the contrast medium continuously includes the higher harmonic wave component over a wide frequency band, in addition to the fundamental wave component corresponding to the fundamental frequency f0. In contrast to this, the response signal 2 from the tissue is divided into a fundamental wave component 2a of the fundamental wave frequency f0 and a higher harmonic wave component 2b of the double higher harmonic wave 2f0. The higher harmonic wave component 2b is not so strong in the case of the shallow part, but it is very strong in the case of the deep part, and it is stronger than the response signal 1 of the contrast medium near the double higher harmonic wave 2f0. This is because the higher harmonic wave component 2b included in the response signal from the tissue is caused by the nonlinear effect in the propagation of the ultrasonic wave within the tissue as mentioned above, so that the propagation length is increased toward the deep part separated from the probe. Accordingly, even when the double higher harmonic wave component 2f0 is uniformly extracted and the response signal from the contrast medium is emphasized, as in the conventional method, the higher harmonic wave component 2f0 of the tissue is also emphasized as well, except at shallow positions, so that the definition of a enhanced-contrast image cannot be improved.
Therefore, an object of the present invention is to distinguish the higher harmonic wave component included in the response signal from the contrast medium from the higher harmonic wave component included in the response signal from the tissue, and to improve the definition of the enhanced-contrast image.